Diesel engines have long been direct injected engines wherein fuel is injected at or near top dead center (TDC) directly into the combustion chamber. Many gasoline engines are now being developed as direct injected engines in order to take advantage of the efficiencies associated with direct injection. Direct injected gasoline engines are now possible due to advances in injector technology as well as advances in engine control strategy. However, direct injecting gaseous fuels such as natural gas or hydrogen have not yet gained similar feasibility.
One of the practical difficulties with using a gaseous fuel in a direct injected system is providing the fuel at a high enough pressure to be injected near top dead center while providing the necessary volume of fuel, which may be 3000 times the volume of operation on diesel fuel. Typical compressed natural gas (CNG) tanks are designed to contain fuel at approximately 3600 psi. In order to direct inject a gaseous fuel at or near top dead center, fuel must be at approximately 5800 psi. This high pressure is necessary in order to overcome the cylinder pressure under compression as well as to provide enough fuel into the cylinder.
Gaseous fuel has a lower energy density by volume when compared to diesel and gasoline, thus a much higher volume of fuel must be introduced into the cylinder in order to produce the same power. Existing infrastructure, tank technology, and fuel systems are designed around the standard 3600 psi system. Accordingly, there is a need for a gaseous fuel system capable of operating as a direct injected fuel system.